Electronically scanned array (ESA) systems rely on precise phase disparity between radiating elements to produce a directional beam. Especially in aviation and military applications, precise phase and amplitude control of each radiating element is critical to meet point angle, beam width, and sidelobe requirements.
As the power amplifier is driven into the compression region and the output power of the power amplifier is saturated, phase distortion of the power output (AM-PM distortion) becomes significant. The phase of radio frequency (RF) output at the 1 dB compression point is about 22 degree deviated from the phase in the linear region for a 2 W amplifier design. Power output in saturation is less sensitive to input power; however, the output phase becomes much more sensitive to input power. For amplitude taper, the phase of RF output from elements at different power levels in the compression region including saturation may be significantly different as compared to that from elements at much lower power levels in the linear region. The phase imbalance among the elements must be minimized in order to meet high performance system requirements.
Calibration methodology based on a table of calibration values (open loop control) cannot accurately control phase to meet high performance requirements when the amplifier is in the compression region. In the compression region, one dB input power variation could cause a phase shift of about 3-4 degrees. In order to achieve 0.5 degree phase accuracy of a high performance ESA system, the calibration method must have 0.25 dB accuracy to control the RF input power. When the amplifier enters saturation, phase shift per dB accelerates substantially. In practice accurately controlling the phase and amplitude at each element is difficult.